The present invention relates to a method of filming video display screens and to an adapter for controlling a video display screen from a camera during a filming session.
When a video display screen, such as a television screen or computer screen is filmed an effect is produced in which a dark or brightened up band appears on the screen when the film is processed. The image of the screen as it appears on the film also unacceptably flickers.
The problem is due to the mismatch between the rate at which the film is shot, typically twenty four or twenty five frames per second for a film camera, and the rate at which the display screen image is built up. These two rates are asynchronous. The bar which appears on the filmed image of the screen is described in the trade as a xe2x80x98roll barxe2x80x99.
Depending upon the type of display screen in use, the frame rate at which the picture on the video display screen is built up may vary. For example, on a television screen where the picture is made up of two fields of inter-laced lines, one full picture field of 312 lines in the PAL system is built up in one-fiftieth of a second in the United Kingdom or one-sixtieth of a second on a US screen for an NTSC standard field of 262 lines. Computer screens do not use inter-laced fields and the time to build up one full picture can vary between {fraction (1/50)} and {fraction (1/85)} of a second or greater.
The existing solutions to this technical problem are far from satisfactory and require that the film and video frame rates are the same to achieve synchronisation of the video and film frames.
For short shots, a device sometimes known as a roll bar eliminator is used which must be manually adjusted every time the film starts. This device changes the speed of the film to make the film rate and the rate at which a picture builds up on the display the same. This approach causes other problems as the film is no longer running at the normal twenty four or twenty five frames per second. The adjustment is usually only stable for a very short period and it is only suitable for shots of less than a minute in duration. This is because the film transport is free running and will eventually drift out of synchronisation with the video display screen. Moreover it is only possible with this solution to correct for one video display screen in each film shot.
GB-A-2 029 596 in the name of the Secretary of State for Defense describes an example of apparatus for synchronously transferring TV pictures on to cine film. This device uses a rotary shutter blade which rotates at one rotation per three TV field scans. The control system synchronises a shutter opening signal with the TV video signal so that the shutter opens at the beginning of a field scan.
For major cinema projects it is known to modify each of the video display screens themselves so that the rate at which pictures build up is the same as the filming rate of the camera. This is done by the use of a standards converter. This usually involves employing an engineer to adjust each screen to be incorporated in any shot and considerably inhibits the director""s ability to vary the film rate to gain appropriate effects during filming.
In order to resolve this technical problem, the present invention provides a device which does not require any adjustment to the frame rate of the camera or the normal video signal of the screen. In accordance with the invention a film camera is supplied with just one complete field of video information for each frame of film.
The present invention provides a device for use with a film camera which has a shutter and output means for providing an output timing signal responsive to the state of the shutter, said device being provided to enable filming of a clear image of a video display screen having an input for a video signal; said device supplying a modified video signal in response to said output timing signal to enable display of only one complete field of video information on the video display screen for each frame of film.
Preferably the device blanks the video signal other than for a period dependent on a duration of one field. This duration can be set by a selector switch or measured automatically by the device, allowing the same device to be fitted to different types of video display screen.
The device preferably comprises means for generating a blanking signal which is fed to the video signal input, and means operative in response to said output timing signal to remove the blanking signal for said one field period.
The approach of the present invention has the advantage that it allows the camera to be run at any desired rate, that several video display screens can easily be included in a single shot, and that the synchronisation can be maintained indefinitely.
Other aspects and advantages of the invention are defined in the appended claims.